1. Field of the Invention
The present invention relates an apparatus for controlling a valve timing in an internal combustion engine, wherein the opening or closing timing of an intake or exhaust valve is suitably controlled in accordance with engine operating conditions.
2. Description of Related Art
Japanese Unexamined Patent Publication No. 60-243308 discloses a mechanism for controlling a rotating phase difference between a camshaft and a crankshaft in an internal combustion engine, the mechanism being provided with a first helical splined shaft fixed to the camshaft and defining on an outer surface thereof a helical spline, a second helical splined shaft fixed to a timing pulley of the camshaft and connected by a belt to a timing pulley fixed to a crankshaft and defining on an outer surface thereof a helical spline, and a timing piston having a spline which is in mesh with the helical splines of the first and second helical splined shafts. The timing piston is connected to a mechanism for obtaining a movement of the timing shaft in parallel to the direction of the axis of the camshaft, to thereby cause the first and second helical splined shafts to be relatively rotated, and thus allow a rotational phase difference to be obtained between the timing pulley, connected to the crankshaft via a belt-pulley mechanism, and the camshaft. The mechanism for obtaining the movement of the timing piston is constructed by an electric motor for obtaining a rotational movement, and a mechanism, such as a ball nut, for transforming the rotational movement from the motor to a linear movement in parallel to the direction of the camshaft. The mechanism for transforming the rotational movement into a linear movement comprises a worm member on an output shaft of the electric motor, a pinion in mesh with the worm, a first slider sleeve defining at an outer surface thereof a gear portion in mesh with the pinion, a second slider sleeve in screw engagement with the first slider, a pin for preventing a rotation of the second slider sleeve about its own axis, and a ball bearing for connecting the second slider with the timing piston while allowing the timing piston to be rotatable with respect to the second slider sleeve. The rotation of the output shaft of the motor causes the pinion to be rotated via the worm, and the rotation of the pinion causes the first slider to be rotated about its own axis, which causes the second slider to be moved axially along the direction of the axis of the camshaft, because the second slider is screw-engaged with the first slider and a rotation of the second shaft is prevented by the pin. The axial movement of the second slider in parallel to the axis of the camshaft is transmitted to the camshaft, and thus a relative angular displacement between the crankshaft and the camshaft is controlled.
The camshaft is provided with cams for obtaining a lifting movement of the respective intake valves and exhaust valves, which causes a rotating torque as a reaction force to be generated in the cam shaft. The direction of the reaction force is such that the rotation of the camshaft is relatively delayed with respect to the crankshaft. Namely, even when the valve timing control device is operated such that the camshaft is rotated in a direction that is advanced with respect to the crankshaft, a reaction force will be generated in the crankshaft in a direction in which the rotation of the camshaft is delayed with respect to the crankshaft. To maintain the camshaft at an angular position that is advanced with respect to the crankshaft, a means of preventing the camshaft from being returned in the direction in which the rotation of the camshaft is delayed with respect to the crankshaft must be provided. The worm gear arranged between the rotating drive motor and the slider in the prior art allows only one directional transmission of the movement from the electric motor to the slider. Namely, any transmission of the movement in the reverse direction, i.e., the direction from the slider to the motor, is prevented, and this allows the obtained relative rotational angle relationship between the crankshaft and the camshaft to be maintained unchanged even if no provision is made therefore.
In the prior art, the slider moves the timing piston in the direction parallel to the axis of the camshaft, to obtain a relative rotating movement of the first and second helical splines in spline engagement with the first and second helical splines. In this construction, the timing piston is connected to the slider via a ball bearing assembly, which allows the timing piston to rotate about the slider while the axial movement of the slider along the axis of the camshaft is transmitted to the timing piston. In the prior art, however, a rigid connection is obtained between the slider and the timing piston. Namely, the ball bearing assembly is provided with an inner race press fitted to the timing piston and an outer race press-fitted to the slider. Such a rigid connection between the slider and the timing piston in the prior art makes it difficult to obtain a smooth movement between the slider and the timing piston, when there is a misalignment of the axis of the slider and the axis of the camshaft in a tolerance range, and as a result, a smooth control of a mutual positioning of the slider and the camshaft cannot be obtained.
Furthermore, the prior art construction is disadvantageous in that a desired amount of lubricating oil cannot be supplied to the worm gear mechanism and the ball nut mechanism, each having parts which are mutually engaging, so that a smooth movement between these parts is sometimes lost.
Still further, the prior art construction is disadvantageous in that the lubrication oil for the worm gear mechanism and the ball nut mechanism is often accumulated therein when the engine is stopped. The viscosity of the oil is increased during low temperature condition, and the high viscosity of the oil thus accumulated during a stoppage of the engine at a low temperature can make it difficult to ensure that the worm gear and/or the ball nut mechanism are moved smoothly, and thus a desired control of the mutual position of the camshaft with respect to the crankshaft cannot be obtained, and accordingly, the cold engine cannot be easily started.